The present invention is particularly applicable for use with linear detectors that use a conical field of view, such as two spaced cameras, at the exit position of a displacement guide roll in a web guiding system, wherein a guide roll is tilted about a lower spaced pivot point to adjust the angular disposition of the moving web to guide the web in a centered position through the system, and the invention will be described with particular reference thereto; however, the invention has much broader applications and may be used for various linear detector systems such as laser detectors, LED array detectors, and other non-contact detectors wherein the edges of the web or strip are detected and their relative sensed positions are employed to determine the lateral displacement of the web from the desired centered position. In these types of web guiding systems, a camera views each edge of the moving strip and uses the sensed strip edge positions in an attempt to determine the lateral position of the moving web. This is accomplished by combining the detected or sensed positions of each edge. To adjust the web from its actual position to a desired centered position, an error signal is created. The magnitude and polarity of the error signal is employed for the purpose of adjusting the actual position of the strip by shifting the strip in a direction necessary to reduce the error signal and, thus, the difference between the actual web position and the desired web centered position. The magnitude of the error generally controls the velocity or rate of position correction. Several systems using cameras, lasers, ultrasonic transmitters, LED arrays and similar edge detecting devices are known in the art. As background information, U.S. Pat. No. 5,058,793 is incorporated by reference herein. This patent shows how a position correcting error signal is employed for the purpose of correcting the centered position of a moving web. This system is only representative in nature and is referenced so that details of the actual position correcting system itself need not be repeated herein. The present invention relates to an improvement involving a tilt compensating system or circuit for use in a position correcting device that employs a device for creating an error signal that not only considers the lateral spacing of the web edges, but also the amount of tilt of the strip. This error signal with tilt compensation is used for strip or web correcting in a web conveying and guiding system.
Spaced cameras viewing the opposite edges of a moving web have a conical field of view which intercepts the spaced edges of the web. The camera detectors "see" only a one dimensional lateral view or shadow of the respective parallel edges of the web. The camera can not determine the perpendicular spacing of the web from the camera or the difference between perpendicular positions of the web edges. This difference is created when the web is tilted from the normal plane of the detecting system. Thus, the detected or sensed edges obtained by the conical field of view of the spaced cameras in a detecting system is only the orthogonally projectable or viewable one dimensional lateral position of the respective parallel edges. There is no economical scheme for the spaced cameras to detect tilt; therefore, when the web is tilted it appears that the web is off center more or less than is the situation, since, for example, an edge tilted closer to the camera can block more of the field of view than the edge which is tilted further from the camera. Consequently, if the web is indeed perfectly centered in the desired controlled position for the moving web, tilting of the web will cause the error detector to create an error signal inaccurately indicating to the position controlling system that the web is off center. Thus, centerline correction is somewhat complicated. The tilting of the moving web distorts the camera view for detecting the actual position of the moving web. This is an inherent deficiency in most non-contact width detector systems that create an error signal indicative of a lateral offset position of the moving web. The discrepancy caused by tilt is less when the web is actually centered and is more pronounced as the web is actually off center. The inaccuracy caused by tilt is more pronounced as the strip is closer to the cameras. For that reason, detector systems have normally been spaced a greater distance from the actual web to minimize the discrepancy caused by tilt of the web. When the non-contact detectors are 20-40 inches from the moving web, the tilt error is significant. By increasing the spacing to a value in excess of 50 inches, the tilt error is less troublesome. Consequently, there is a tendency to space the web substantially from the detectors. This greater spacing causes other problems, such as lack of physical space available for the system and atmospheric contamination in the area between the web and the cameras. For these reasons, there has been substantial difficulties encountered in use of cameras with conical field of view for use in the exit span of the web detector system. This same problem of tilt error is present in other detector systems of the type using a conical field of view or even in an LED array, such as the ACCUWIDE series of non contact measurement sensors sold by North American Manufacturing Company of Cleveland, Ohio. All of these detector systems for creating a web displacement error signal have the difficulty that they are somewhat inaccurate due to the inability to discriminate between offset of the web and tilt of the web.